Beware of people suggesting their personal favorite language. There are many languages worth knowing. My personal favorite is the Lisp family and particularly Clojure. Pick up a book like SICP or any of the Clojure books. It can really give you a new perspective.
I suggest buying the Seven Languages book by Bruce Tate. There is no wrong choice, because learning any language will make you better.
I suggest buying the Seven Languages book by Bruce Tate. There is no wrong choice, because learning any language will make you better.